1. Field of the Invention
The present invention relates to a free cutting steel for machine structural use having improved properties by containing sulfide inclusion particles with controlled aspect, size and distribution.
One aspect of the present invention concerns a free cutting steel of excellent machinability strengthened by controlling aspect, size and distribution of MnS-based inclusion particles in the steel matrix into limited scopes.
Another aspect of the invention concerns a free cutting steel of a high rolling-contact fatigue strength improved by controling aspect of sulfide inclusion particles in the steel so that the majority of the relatively large particles may not be extremely elongated and by lowering areal percentage of alumina cluaster in the matrix cross section.
The free cutting steel for machine structural use of the present invention covers carbon steel, manganese steel, nickel-chromium steel, chromium- molybdenum steel, mickel-chromium-molybdenum steel, manganese-chromium steel, molybdenum steel and nickel-molybdenum steel.
2. State of the Art
It has been well known that some elements such as sulfur, tellurium and lead are useful for improving machinability of steels, and free cutting steels which have increased machinability by adding one or more of these elements to carbon steel or low alloy steel are widely used.
Demand for better machinability of steel, however, has been not completely satisfied, and various industries have been seeking further improvement in machinability of steel.
The inventors found the fact that steel for machine structural use containing suitable amounts of Te and S exhibits not only increased machinability but also decreased anisotropy in mechanical properties and good formability in cold forging. There has been a need for improvement of rolling-contact fatigue strength of such a kind of free cutting steel.